Host-specific AB-toxin production by germinating spores of Alternaria brassicicola is induced by a host-derived oligosaccharide

The phytopathogenic fungus Alternaria brassicicola is the causal agent of black spot disease of Brassica plants and produces a host-specific protein toxin named AB-toxin. AB-toxin is released from spores germinating only on host leaves. In this study, we found a factor for inducing AB-toxin production that is released from host leaves after spore germination has begun. GC and GC-MS analyses of the compound purified by gel filtration chromatography demonstrated that it is an oligosaccharide of 1.3 kDa. This is the first evidence that host-specific toxin production by germinating spores of a fungal pathogen is induced by recognition of host-derived factors.